In the process of xerography as disclosed for example in Carlson U.S. Pat. No. 2,297,691, a latent electrostatic image of the material to be reproduced is formed on a xerographic plate. Development of the image on the plate is effected by a toner comprising a suitable pigmented or dyed electrostatic powder carried on a granular material which functions to provide mechanical control of the powder and to carry the powder to the image surface. The granular material carrying the toner is transported from a suitable reservoir to the xerographic plate and allowed to flow down over the plate. As the powder coated carrier particles roll or tumble down over the xerographic plate carrying a latent electrostatic image, toner particles are pulled away from the carrier by the charged latent image and deposit on the plate to form a powder image while the partially denuded carrier particles pass off of the plate and are returned to the reservoir.
In order to replenish the toner particles on the carrier material, additional toner particles must be supplied to the developer mixture in proportion to the amount of toner deposited on the plate. Accordingly, toner dispensing means must be provided for supplying toner in the correct amount. If insufficient toner is supplied, the prints produced by the apparatus have low contrast images. If on the other hand too much toner is supplied, the background of the copies produced may have a gray or muddy appearance. Thus, in order continually to obtain prints of good quality, the quantity of toner powder in the developer mixture must be held reasonably constant by the addition of toner to the developer mixture in proportion to the amount of toner used up.
As the toner is a very fine electrostatic powder it is difficult to handle and to dispense uniformly in a precisely controlled amount. The powder tends to agglomerate and cake so that it does not flow uniformly in proper amount from the dispenser. Apparatus has heretofore been provided with a small paddle or stirrer for manually stirring the toner in the dispenser from time to time in order to break up any caking. However, because of the black and adhesive nature of the toner this is a very messy operation.